1. Field of the Invention
This invention relates to a method for reforming a hydrocarbon feedstock such as naphtha. More particularly, the invention is concerned with a method for improving the yield selectivity of a multibed catalytic reformer operating below design capacity.
2. Discussion of the Prior Art
The term "reforming" is well known in the petroleum industry and refers to the treatment of gasoline or fractions thereof to improve their anti-knock characteristics. The reforming process involves many reactions not all of which are entirely understood or even known. The primary known reactions comprise controlled or selective aromatization and cracking, the former including dehydrogenation of naphthenic hydrocarbons to aromatics and the cyclization of straight chain or mildly branched chain aliphatic hydrocarbons of at least six carbon atoms to form aromatics. Other reactions occurring during reforming include isomerization, both of aliphatic hydrocarbons and of naphthenic hydrocarbons containing five and six ring carbon atoms, hydrogen transfer reactions, alkyl transfer reactions, and the like.
In the reforming process of naphthenic compounds, both cyclohexanes and cyclopentanes are converted to aromatic compounds to obtain high octane reformate. In reforming, the cyclohexanes can be converted to aromatics by a simple, clean dehydrogenation reaction, while the cyclopentanes have to be first isomerized and then dehydrogenated to yield aromatics. In comparison with cyclohexanes, reactions of cyclopentanes to aromatics are not only slower, but also lead to undesirable side reactions as for example, cracking to light gases, resulting in lower reformate yield and poorer process performance.
At times of decreased reformate demands, it is necessary to operate reformers below design capacities. However, even when operating below design throughput, the catalyst fills of the reforming units are at the same level found at design throughput. Thus, most units operating below design throughput are holding more catalyst than they require, and are therefore running at lower than optimum space velocity and reactor temperature.